From "Doctor Who" to "Dollhouse," these are the series that told their greatest TV stories in worlds far away from our own.

It hardly needs mentioning, but not all science-fiction is created equal. Even as a term, it’s difficult to pin down exactly what constitutes sci-fi, especially in a medium like TV that can change so much from week to week or season to season. So when we took stock of the best TV shows from the past two decades, we turned to one unimpeachable source as a guide for what we should include in our list.

Thanks, Roman.

That’s not to say that a few of the shows might have an odd sword or two, but most of what we tried to single out here imagines a world markedly different from our own. Some take place in reimagined futures, while others take a sharper focus on the present with some key, otherworldly changes.

Given the wealth of shows in this genre, we opted not to include animated or unscripted series in this particular list. (Otherwise “Rick & Morty,” “Futurama” and “Samurai Jack” would definitely rank high here.) As usual, we’ve also kept these picks to shows that premiered after 2000.

So, follow us down the rabbit hole to some of the most thrilling, thought-provoking —and in many cases, utterly horrifying — alternate realities that TV has brought us in recent years.

20. “The 4400”

“The 4400”

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First off: Anyone who watched “The 4400” during its initial run on USA was likely not surprised to see Mahershala Ali — or Mahershalalhashbaz Ali, as he was known then — eventually become an Oscar-winning actor. As Richard Tyler, one of the 4,400 individuals plucked from history to be deposited in the year 2004 with mysterious abilities, Ali was one of the show’s most engaging performers, an empathetic presence who sold us on the show’s emotional drama while we waited to find out exactly how and why this seemingly random collection of strangers had been transported to the future. Not every plot twist of this series worked, but the mysteries kept us hooked during its run, as the show managed to never lose sight of its human stories even while taking on large-scale sci-fi drama.

19. “Sense8”

“Sense8”

Murray Close/Netflix

The science part of the science fiction on “Sense8” is merely the entry point to a fantastic story of human nature. Eight individuals born around the world on the same day realize they have a psychic connection that lets them feel and experience things simultaneously but also take over each other’s bodies in order to help them through trying situations. This wholly unique device demonstrated how we are not alone as long as there is acceptance of the universal human experience.

Batshit nuts at times, the show has created indelible scenes in which the Cluster or Sensates experience events around the globe as if they’re in the same room, whether it’s sex (ah, the famous orgy scenes), swimming, executing a jailbreak or even childbirth. The Wachowskis’ artistic eyes have created some of the most breathtaking images seen on TV, and their genius shines when it comes to crowd scenes where it seems like everyone is filled with the same, contagious elation. There’s no party like a “Sense8” party, and it’s no wonder that the emotional force of this series inspired its devoted fanbase to petition for the show to continue after Season 2’s cliffhanger ending. It’s a testament to the voice of that global cluster that Netflix finally saw sense (ha!) and announced a series-capping movie that would round out the story.

18. “The Middleman”

Given how hard it is to explain exactly what was going on with the ABC Family sci-fi series, we understand why it might have been canceled. But the cheerful homage to decades of pop culture was a delight to watch, especially thanks to charming lead performances by Natalie Morales and Matt Keeslar. “The Middleman” was dense with references, but it also kept its central characters fun and relatable as they dealt with no shortage of absurd scenarios, drawn from the widest reaches of sci-fi and fantasy tropes.

17. “The 100”

Sometimes, you watch a post-apocalyptic drama and you think, “Hmmmm, feels like they pulled their punches to some degree.” This is not true of “The 100.” One of the most brutal shows we’ve ever seen, “The 100” also happens to be rich with big sci-fi ideas, even as it pits its characters against each other in semi-regular battles to the death. The CW series’ commitment to coming up with shocks and surprises has proven fascinating to behold, even as it finds new depths to its mythology.

16. Terminator: The Sarah Connor Chronicles

“Terminator: The Sarah Connor Chronicles”

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Arguably, the “Terminator” franchise is only really a franchise because of “The Sarah Connor Chronicles.” Sure, the first two films reaped in profits and awards hand over fist, but nothing since then has stuck. “Terminator 3: Rise of the Machines,” “Salvation,” and “Genisys” all flopped stateside, with each film making significantly less money than the last, and none earned enough raves to pass the test of time.

But “The Sarah Connor Chronicles,” which lasted 31 episodes over two seasons, is the third piece of the puzzle; the missing ingredient needed to make fans of the franchise believe there doesn’t have to be an end. Where the later films have failed to sustain a sense of wonder and excitement, the Lena Headey-starring series picks up right after “T2” and never looks back. Solid action, an aptly winding plot, and a strong lead turn make this one worth remembering — and successful enough that many fans still do. It makes sense: After all, “Terminator” always was Sarah’s story.